When storing liquids, the problem arises that escaping gaseous substances can entrain the liquid so that, apart from the pollution of the pertinent apparatus, also sources of danger may occur. For instance, when storing oil in large-volume tanks, there is a risk of oil being entrained because of ascending air or gas bubbles. This problem is especially relevant to tanks in which two preconditions are fulfilled:                The tank has a quite large construction height and introduced liquid is supplied to the lower portion of the tank. In this way, a more or less strong degasification of the oil results from the pressure drop when the liquid moves upward. Moreover, large gas bubbles entering the venting system at high velocity may form.        The ceiling of the tank is arranged very closely to the stored oil. Especially in the case of floating roof tanks having ceilings located on the oil surface the problem of the ascending gas bubbles entraining the oil into the vent openings arises.        
So far complex liquid separating systems have been used in process engineering. These systems are frequently provided either with movable elements or with complex valves.
In those systems including valves or movable elements the risk of pollution and clogging, especially by viscous fluids, occurs. Then the venting function cannot be guaranteed any more.